Hierarchical modulation for hierarchically superimposing individually mapped signals is one of the known methods for simultaneously transmitting a plurality of data streams having different data priority levels and transmission speeds (Non Patent Literature 1 described below). Layers in the hierarchical modulation are usable as independent lines. Note that, among the layers of the hierarchical modulation dealt with here, a first layer has the highest line quality (the lowest bit error rate) and lower layers have lower line qualities (higher bit error rates).
In general, when two streams are multiplexed by using hierarchical modulation, each of the layers is separated into either a high-reliability line (a first layer) for QPSK (Quadrature Phase Shift Keying) or the like and a low-reliability line (a second layer) for QAM (Quadrature Amplitude Modulation), or the like, that is superimposed on the first layer. In such a case, there is a problem in that the line qualities of the layers in conventional hierarchical modulation are fixed and the amount of freedom allowed when designing a circuit is low when hierarchical modulation is used.
A measure against the problem is disclosed in Non Patent Literature 2, listed below. The measure disclosed is a technique for changing sizes (constellation sizes) of signal points of layers during superimposition in order to adjust line quality.